Final answer:
Failed endodontic treatments are often associated with the bacterium Enterococcus faecalis, which is resilient and capable of forming biofilms, making it difficult to remove from the root canals.
Step-by-step explanation:
The bacterium primarily associated with failed endodontic treatments is Enterococcus faecalis. This bacterium is known for its ability to persist within the root canals of teeth after endodontic therapy, which can lead to infections. E. faecalis is resistant to many common antibiotics and has the ability to form biofilms, making it particularly challenging to eradicate from the endodontic space.